The present invention is particularly useful for monitoring a patient's central venous pressure. Accordingly, that application will be discussed in the following detailed description. However, the present invention may have application in situations where other bodily fluid pressures are to be measured. The scope of the present invention should therefore be determined by the attached claims and not the following detailed description.
Central venous pressure (CVP) is the pressure of blood in the right atrium or vena cava of a human heart. A patient's CVP provides important information about the patient such as: (a) the patient's blood volume; (b) the effectiveness of the patient's heart as a pump; and (c) the patient's vascular tone. Therefore, in combination with a clinical assessment of the patient, a health care worker will often monitor the patient's CVP, and particularly any upward or downward trends in CVP, to determine whether and what type of intervention is appropriate for the patient.
CVP is normally measured in terms of centimeters or millimeters of water pressure. A typical pressure in the right atrium is 0 to 4 cm H.sub.2 O, while pressure in the vena cava is approximately 6 to 12 cm H.sub.2 O.
One method of measuring CVP is to insert a catheter into an arm or leg vein or the subclavian vein and thread the catheter through the vein into the vena cava close to the right atrium. FIG. 3 depicts a catheter inserted through the subclavian vein into the vena cava. A manometer or other pressure measuring device is then attached to second end of the catheter through intravenous (IV) connecting tubing. If a manometer is used, pressure in the vena cava displaces or equilibrates with the pressure exerted by the column of fluid in the manometer and the point at which the fluid in the manometer settles is the recorded as the CVP.
In a modern hospital setting, the pressure measuring device is typically a pressure transducer which generates an electrical signal corresponding to the pressure at the end of the IV connecting tubing. Such pressure transducers generally comprise a diaphragm so arranged that the diaphragm is measurably displaced by the pressure in the IV connecting tubing. The electrical signal generated by the transducer is proportional to the displacement of the diaphragm. Because displacement of the diaphragm is proportional to CVP, the magnitude of the electrical signal can thus be empirically related to CVP. Properly calibrated with equipment for analyzing these electrical signals, a pressure transducer allows the health care worker to accurately measure and monitor trends in CVP.
In order to obtain an accurate measurement of CVP, the zero point of the manometer or a given measuring point on the pressure transducer must be closely vertically aligned with a desired point on the patient such as a mark corresponding to the patient's right atrium and/or vena cava. If the vertical level of the point at which pressure is measured is below the right atrium, the CVP reading will be too high. On the other hand, if the vertical level of this measuring point is higher than the right atrium, the CVP reading will be too low.
Perhaps even more importantly, if the vertical level of one of the right atrium and the measuring point changes over time while the other of these vertical levels remains the same, CVP may appear to trend upwardly or downwardly when actual CVP has remained constant or, alternatively, an actual upward or downward trend in CVP may be masked by the change in measured CVP. This may result in the health care worker providing inappropriate intervention. Accordingly, consistency in aligning the pressure monitoring device in relation to the right atrium over time is perhaps more important than accurately aligning the pressure monitoring device with the right atrium.
The consequences of not consistently aligning the pressure monitoring device with the right atrium of the heart can be quite serious. For example, if: (a) a first CVP measurement is taken at time t.sub.1 ; (b) the patient subsequently moves or is moved so that the patient's right atrium is lower than at time t.sub.1 ; (c) by time t.sub.2 the patient's actual CVP raises, indicating higher blood pressure; and (d) a second CVP measurement is taken at time t.sub.3, the second CVP measurement may not reflect the rise in CVP that occurred between time t.sub.1 and time t.sub.2. The health care worker may mistakenly assume that CVP is constant and no intervention is necessary, while in fact the higher actual CVP may indicate third spacing of bodily fluids caused by increased blood volume and pressure. In this case, the appropriate intervention, i.e., halting the flow of intravenous fluids into the blood stream, may not be taken. Serious problems may thus be hidden by a pressure transducer that is improperly vertically aligned with the right atrium of the heart.
It is therefore essential that the pressure measuring device be consistently and accurately aligned with the right atrium of the heart so that actual CVP and upward and downward trends in CVP are accurately monitored.